This invention relates to cathode ray tube construction and more particularly to an improved combination means for suppressing deleterious arcing in the region of the electron gun assembly.
In the continuing progression of cathode ray tube technology, there has been a trend toward the utilization of higher screen potentials along with the miniaturization and compaction of associated electron gun structures encompassed within envelope neck portions of smaller diameters. Consequently, spacings between related electrode components in the electron gun structure have been reduced in keeping with design parameters. The closeness of these inter-electrode spacings in conjunction with the high voltage differentials existent within the tube, and the presence of possible contaminants, increases the possibilities of arcing within a tube structure.
It has been conventional practice in cathode ray tube construction to have a conductive coating disposed on the interior surface of the funnel portion in a manner extending from the vicinity of the screen into the forward region of the neck portion. This coating, which usually has a high positive electrical potential applied thereto via connective means through the funnel wall, serves as a connective medium conveying a high potential of substantially the same value to both the screen and the terminal electrode of the electron gun assembly oriented in the neck portion of the tube. Thus, the condition is present for the possibility of a spark discharge between the terminal electrode and the adjacent lower voltage electrodes in the gun assembly, especially in the presence of aggravating elements such as sublimation deposits, foreign particles, and minute projections extending into the inter-electrode spacings. While considerable effort is expended during tube manufacturing to minimize the factors contributing to arcing, the utilization of anode potentials in the order of 30 KV and higher makes the possible presence of minute arcing conditions factors of extreme importance. Arcing or dielectric breakdown within the cathode ray tube has always been an undesired probability and has been found to exhibit destructive intensities of 100 amperes or more. With increased employment of solid state components in television and allied display devices, arcing within the cathode ray tube can produce catastrophic effects on the components in the operating circuitry. Additionally, an arc discharge may damage the internal structure in the tube and sublimate deleterious metallic deposits in the region of the gun structure.
Cleanliness, precision, and care in the manufacturing process are ever-continuing procedures to combat the materializing of conditions conducive for arcing. Nevertheless, human factors, processing sublimates and manufacturing tolerances sometimes combine to produce the undesirable situation. The discrete use of high resistance coatings on interior areas of the funnel has been tried. For example, one such technique is that disclosed by A. V. de Vere Krause in U.S. Pat. No. 2,829,292, wherein a band of resistive coating was internally applied to substantially the juncture region of the funnel and neck portions of the tube envelope to provide a high resistance area to limit the spark discharge current in the region of the electron gun. However, it was found that getter and other sublimation deposits within the tube tended to bridge the resistance coating thereby decreasing the intended benefit. Additionally, particles of the resistive coating tended to loosen upon insertion and placement of the electron gun. Since the minimization of arcing in present-day color cathode ray tubes is assuming ever increasing importance, it is a prime concern in tube manufacturing to achieve an expedient means for adequately controlling the arcing environment within the cathode ray tube per se.